Hygrometry is the branch of science that deals with the measurement and study of water vapor in the air. It is an important aspect of meteorology and plays a crucial role in various industries such as agriculture, HVAC systems, and manufacturing processes.

Water vapor in the air can greatly affect the comfort, health, and productivity of individuals, as well as the performance and efficiency of various processes and equipment. The measurement and control of humidity levels are essential in many applications to ensure optimal conditions.

Here is a detailed explanation of why option B, "water vapor in air," is the correct answer for the question:

1. Definition of Hygrometry:
- Hygrometry is the scientific study and measurement of humidity, which refers to the amount of water vapor present in the air.
- It involves the measurement and analysis of various parameters related to humidity, including absolute humidity, relative humidity, dew point, and specific humidity.

2. Importance of Water Vapor in Air:
- Water vapor is an important component of the Earth's atmosphere and has a significant impact on weather patterns, cloud formation, and precipitation.
- It affects human comfort and health, as high humidity levels can cause discomfort, while low humidity can lead to dry skin, respiratory problems, and static electricity.
- In industries, humidity control is crucial for processes such as manufacturing, storage, and preservation of products. For example, in the textile industry, humidity levels need to be controlled to prevent the loss of moisture or shrinkage of fabrics.

3. Measurement of Water Vapor:
- Hygrometers are devices used to measure humidity levels. They work on different principles, including the use of hair or synthetic materials that change their length or electrical properties in response to humidity changes.
- Hygrometers provide readings in various units, such as percentage relative humidity (%RH) or grams of water vapor per cubic meter of air (g/m³).

4. Applications of Hygrometry:
- Meteorology: Hygrometry is essential in weather forecasting and climate studies. It helps in understanding atmospheric moisture patterns, cloud formation, and predicting precipitation.
- Agriculture: Humidity control is crucial in greenhouses to create optimal growing conditions for plants. It helps in preventing diseases, controlling transpiration, and maximizing crop production.
- HVAC Systems: Humidity control is important for maintaining comfort and indoor air quality in buildings. It helps in preventing mold growth, reducing static electricity, and optimizing energy efficiency.
- Pharmaceuticals and Electronics: In these industries, precise humidity control is crucial for product stability and preventing moisture-related damage.

In conclusion, hygrometry is the branch of science that deals with the measurement and study of water vapor in the air. It plays a crucial role in various industries and applications where humidity control is essential for optimal conditions, human comfort, and the performance of processes and equipment.